This disclosure relates to polycarbonate compositions, methods of manufacture, and uses thereof to form articles.
Polycarbonates compositions are useful in the manufacture of articles and components for a wide range of applications, from automotive parts to electronic appliances. Despite their many advantages, one property that has proven difficult to achieve in some polycarbonate compositions is weld line strength. In the molding of parts from thermoplastic materials, it is common to use a mold design in which molten plastic flowing in one direction meets molten plastic flowing from another direction as the mold fills. Where the two masses meet, the streams fuse together and form a bond at the juncture. The zone where the materials join is often referred to in the industry as the weld line or knit line. The strength and impact of a molded part at the weld line is usually lower than in other sections of the part. This problem is exacerbated when the part is complex, such as those processed in a mold in which several weld lines are formed in the same part. Reduced weld line strength imposes severe limitations on the design of parts. In many cases the dimensions of the entire part are controlled by the weld line strength, and the need to have a safety factor to compensate for weakness in this area can force the designer to make parts larger than necessary, or to substitute a higher performance (and more expensive) polymer than would otherwise be needed.
A variety of approaches have been used in a attempt to improve weld line strength in polycarbonate compositions, particularly compositions comprising impact modifiers such as acrylonitrile-butadiene-styrene (ABS). U.S. Pat. No. 3,988,389 to Margotte et al. discloses that the weld line strength of a polycarbonate/ABS blend improved with use of a specific graft ABS formulation having a narrowly defined graft ratio, rubber particle size, rubber content, and the like. British Patent 1,182,807 describes a blend of a thermoplastic polycarbonate and a poly(methylmethacrylate) that optionally includes a minor proportion of a “rubbery polymeric material” such as styrene-butadiene rubber (SBR), nitrile rubbers, and graft polymers, primarily rubbery acrylate copolymers. U.S. Pat. No. 5,128,409 to Gaggar discloses that improve weld line strength is achieved using a combination of polycarbonate, a high rubber graft ABS, and up to about 20 wt. % of a poly(methyl methacrylate).
While suitable for their intended purposes, there nonetheless remains a continuing need for polycarbonate compositions with even more improved weld line strength, particularly compositions having improved weld line strength together with good impact strength, softening temperature, and/or good flow properties.